1. Field of the Invention
The present invention relates generally to magnetic media drive apparatus, more particularly to magnetic media data storage peripheral apparatus for computer applications, and more specifically, to a device and method for the precise positioning of a read/write head with respect to the magnetic medium.
2. Description of the Related Art
Computers generally use magnetic media devices for the storage of software programs and data. Various types have been devised, including hard disk, floppy diskettes, optical and magneto-optical disk, and magnetic tape drives. Information is recorded onto and retrieved from the magnetic medium by using a head device (magnetic or laser) to create or sense, respectively, flux transitions on the medium that are representative of the binary "1's" and "0's" that form the digital information (such as a software program or user-created data).
While the invention disclosed will be recognized by those skilled in the art as applicable to any of these types of storage apparatus, the invention will be described in an embodiment for a streaming magnetic tape drive head positioning application. Examples of such devices are described in commonly assigned U.S. Pat. Nos. 4,717,866, 4,747,004, and Re. 33,661. A basic description of streaming tape drive technology can be found in STREAMING, Copyright 1982, Archive Corporation, Library of Congress Catalog No. 82-072125.
The magnetic tape used in streaming magnetic tape cartridge drives is nominally one-fourth of an inch in width and of various lengths, commonly from over two-hundred to six hundred feet per cartridge. Data is recorded in parallel tracks on the tape in serpentine, serial fashion. That is, when one track has been fully read from or written to, the direction of the tape is reversed and the next track used in the opposite direction. State of the art tape drives may create and use forty tracks squeezed across the one-fourth inch width of the tape and hold over one gigabyte of data. In order to read and write the data onto the tape in this fashion, the magnetic head must be able to move transversely across the tape in increments as small as the width of a track and be maintained in the proper orientation while using each track.
Generally, this can be accomplished by a lead screw mounting of the head that is driven by a stepper motor. The head is mounted on a "carriage" that threads onto the lead screw so that for a given amount of rotation of the lead screw, the head is displaced a predetermined distance across the tape. As the number of tracks increases across the fixed width of the tape, the head must be positioned with even greater precision. If the head is not accurately positioned over the proper data track, data transmission errors occur from reading data from the wrong track or writing data onto the wrong track, either of which may result in a loss of the data.
When a tape cartridge is loaded into a tape drive, in order to locate the head to a desired track location on the tape, the head must start from a fixed reference position. Precision alignment calibration of the head has been accomplished in several ways. For example, U.S. Pat. No. 4,476,503 describes the use of the physical edge of tape as a reference starting point to set the write or read gap of the magnetic head to seek a displaced track at a known spacing from the edge.
Another, simpler method is to use a mechanical head carriage "stop" on the lead screw. See e.g., U.S. Pat. No. 4,717,866, lead screw collar 42; or Re. 33,661, collar 32.
The process of so locating the head to a known reference position point is sometimes referred to as "recalibration." When a tape cartridge is loaded into a drive, or when a "reset" signal is received as an instruction from a computer to the drive controller circuitry, the head is located to such a fixed location reference stop prior to moving the head to an appropriate track to perform a read/write function.
A typical head carriage mechanical stop device is shown in FIGS. 1, 2 and 3 (Prior Art). A magnetic head 2 is mounted on a head carriage 4. The carriage 4 rides on a lead screw 6 to raise or lower the head 2 linearly across the width of a magnetic tape (not shown). A stepper motor 8 is connected by gears 10, 12 to the lead screw 6 to rotate the lead screw 6 and thus move the carriage 4 linearly along axis A--A such that head gaps 14 can be brought into preselected alignment with the various tracks of the tape. The lead screw 6 has a stop collar 16 fixedly mounted thereon by clamping screw mechanism 18. Referring to FIGS. 2 and 3, the stop collar 16 is furnished with a protrusion 20, having a face 22, that rotates radially in a fixed orientation orbit about axis A--A. Carriage 4 is provided with a fixed abutment protrusion 24, having a face portion 26. In operation, as the motor 8 turns the gears 10, 12, the lead screw 6 will rotate. With proper rotation, the carriage 4 will drop along axis A--A as shown in FIG. 2 until such position as stop collar protrusion face 22 meets carriage abutment face 26, physically stopping the motion of the carriage 4 (FIG. 3). This anti-rotation device establishes the "carriage stop" as a repeatable reference position point for the head carriage 4 and thus the head 2 with respect to the magnetic tape.
Conventional carriage stop techniques formed by a contact of mechanical features have inherent limitations and problems.
One problem is in maintaining accurate repeatability of the reference position. The protrusions must be manufactured to fairly tight tolerances in order to achieve acceptable repeatability. Since the magnetic head carriage guidance system is reliant upon physical contact, repeatability will be influenced by manufacturing tolerances, dirt, and wear. The repeated impact can eventually loosen the stop collar enough to cause a shift in the reference position. Moreover, repeated impacts can create undesirable and possibly catastrophic wear and tear on associated electrical wiring to the magnetic head.
When contact is made, any additional driving force from the motor cannot produce any further translation of the carriage. The energy supplied is converted to backlash forces and noise. Both the impact itself and the continued force creates wear on the various parts of the carriage guidance system as described above. In time, this will degrade the performance of the entire system. As the carriage guidance mechanism wears, the reference stop position can change and thus can move the magnetic head to a different reference position than that to which it has been initially calibrated at time of manufacture. If the head carriage migrates to such a new reference point, it produces additional errors such as azimuth shift and zenith shift at the head. Because of the microscopic track width, only a minimal change can affect the operational characteristics. Therefore, the primary performance of the tape drive--the storage of retrievable digital data--will be degraded.
Since the conventional carriage stop is form by a contact of two mechanical members, an acoustic noise is produced. Depending in its intensity, this may exceed the allowable noise specifications of the tape drive or, at the least, be undesirably annoying "noise pollution."
When the computer orders a head position recalibration process, the position of the head carriage is unknown. Thus, the carriage stepper motor must rotate to cover the maximum distance possible of carriage motion. Thus, even if the head carriage happened to be in near proximity along the lead screw to the carriage stop, the full recalibration process must be performed. In this event, the motor continues to apply force even when physical abutment features have already come into contact. This not only increases the wear on the components of the carriage guidance system, but also wastes time.
Therefore, there is a need for an improvement in a magnetic head carriage guidance system. More particularly, there is a need for a carriage stop that will have all the functionality of the conventional carriage stop but without the inherent problems of using a physical contact mechanism to establish a reference point.